Design and Comparative Analysis of 90º Hybrids in FR4 and RO4003C Substrates for 5G NR Systems

This work presents the design and comparative analysis of two 90º hybrids for phase-shift applications on OFDM signal processing in the context of 5G NR systems operating at 900 MHz (Band n8). Both designs were validated by electrical simulation and measurements that show two main conclusions: the low-cost (FR4) and the high precision (RO4003C) solutions, as expected. The analysis demonstrates that the physical modeling of the junctions and the fabrication methods shift the simulated 900 MHz FR4 resonance frequency into 917 MHz and the same simulated frequency in RO4003C into only 905 MHz. Nonetheless, in the target frequency of 900 MHz, even with resonance shifts, the FR4 board was able to provide an SXX of -33.5 dB, while its SXY presented an expected imbalance due to the lack of material homogeneity. In contrast, the RO4003C board presented an excellent SXY symmetry, while only an SXX of - 19.1 dB, due to a reduction in trace thickness near the SMA connector. The differential phase shift was measured to be -92.5º on the RO4003C and -91.5º on the FR4 version. These results confirm the reliability and precision of the RO4003C substrate when compared to FR4, even when manufactured in low-cost CNC machines.